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Bacterial evolution in beans could help prevent crop disease

Issue date: 10 September 2009

New discoveries about how diseases spread in bean plants could help researchers to develop new ways to prevent diseases in this valuable food crop.

Scientists have discovered that bean plants' natural defences against bacterial infections could be unwittingly driving the evolution of more highly pathogenic bacteria. The new research published today (10 September) in Current Biology is based on a joint project lead by the University of the West of England in partnership with Imperial College London and the University of Reading.

Significant work was conducted by PhD student, Helen Lovell from UWE who is thrilled to have her first, first author paper published in an international journal.

The study sheds new light on how bacterial pathogens evolve and adapt to stresses from host plants. This information could help researchers develop new ways of tackling pathogens that cause extensive and costly damage to beans and other food crops.

Helen Lovell explains, “As the bacteria die within the plant as a result of the immune response, they release chunks of their DNA which can be taken up by surrounding bacteria. This mechanism allows bacteria to evolve much more quickly than normal, as they gain huge numbers of new genes in one go.

“This work is a leap forward in understanding bacterial evolution. The transfer of large pieces of DNA containing possibly hundreds of genes from one bacterium to another, has only been theorised in plant pathogens and never before demonstrated happening naturally within the plant.

“This work gives an insight into how pathogens evolve within host plants, which may help in developing new methods of disease control to prevent costly damage to bean and other crops.”

The scientists focused on a bacterial pathogen called Pseudomonas syringae, which causes a disease called halo blight, in bean plants. Symptoms include brown spots on the leaves, surrounded by a yellow halo. The disease can cause bean plants to lose their leaves, wilt and die, and is a serious problem for farmers worldwide.

The research team observed that a French bean plant's defensive moves against infection caused P. syringae bacterial cells to 'swap' bits of DNA with each other. When one bacterial cell takes up DNA released by another like this, it is known as genetic transformation. This process, occurring within infected plant tissue, could speed up the evolution of more virulent forms of disease-causing microbes say the researchers.

Professor John Mansfield from Imperial College London's Department of Life Sciences, one of the authors of the new paper, said “In the course of fighting off infection, and putting the invading bacteria under stress, it seems that the plants inadvertently do them a big favour. By causing the bacteria to throw out selected fragments of their DNA, which can then be taken up by other bacterial cells, the plants are effectively stimulating the bacteria to evolve. For disease-causing bacteria, this means that mechanisms meant to disable them could actually contribute to their continued survival.”

When a French bean plant is infected by P. syringae it defends itself by sending a suicide signal to the plant cells surrounding the bacteria. When the affected plant cells die they release antimicrobial compounds that are toxic to the microbes. The toxic environment places the bacterial cells under enormous stress.

The new study shows that along with restricting bacterial multiplication, the release of these toxins seems to stimulate P. syringae cells to cut out small sections of their own DNA containing genes linked to pathogenicity. These gene 'islands' are then thrown out of the bacterial cell, and absorbed and incorporated into the DNA of other bacteria within the plant.

The scientists are not yet sure exactly how the suicide of nearby plant cells brings about this DNA separation and removal, but say their results could have a much wider implication for how scientists understand the relationship between pathogen, host and pathogen evolution.

Dr Dawn Arnold, co author of the study from UWE's School of Life Sciences, concludes, “Although this work involves plant-bacteria interactions it also has a wider significance in that it could lead to a greater understanding of how bacteria evade the immune system of different hosts including humans.”

The research was funded by the Biotechnology and Biological Sciences Research Council (BBSRC).